Novel ground state structures of N-doped LuH3.

Ab-initio crystal structure searches have played a pivotal role in recent discovery of high-Tc hydride superconductors under high pressure. Using evolutionary crystal searches, we predict novel ground state structures of N-doped LuH3 at ambient conditions. We find an insulating ground state structure for LuN0.125H2.875 (~1.0 wt.% N), contrary to earlier studies where assumed structures were all metallic. This insulating behavior of ground state was found to persist up to ~45 GPa. However our crystal structure search revealed a metallic state for an H-deficient variant of LuN0.125H2.875. We study bonding characteristics of important structures by calculating electronic density of states, electronic-localization functions and Bader charges. Our Bader charge analysis shows that insulators have both H+ and H- ions whereas metals have only H- ions. We find that H+ ions are bonded to N atoms via a very short covalent bond. Thus we identify a clear relationship between formation of N-H bonds and insulating behavior of materials. Besides this, we perform crystal structure searches for three more compositions with higher N-content (> 1.0 wt.%). Analysis of electronic properties shows that the ground states of these compositions are insulator.